1. Field of the Invention
The present invention relates to hearing aids. The invention, more specifically, relates to a filter for a hearing aid.
ITE hearings aids generally comprise a shell, which anatomically duplicates the relevant part of the user's ear canal. A receiver is placed in the shell in communication with an acoustic outlet port arranged at the proximal end, i.e. the end of the shell intended to be situated in the ear canal close to the tympanic membrane. The distal end of the shell, i.e. the opposite end, intended to be oriented towards the surroundings, is closed by a faceplate subassembly, connected to the receiver by leads. The faceplate subassembly incorporates a microphone, electronics, a battery compartment and a hinged lid. The microphone communicates with the exterior through a port, which may covered by a grid.
Whereas an ITE hearing aid may be regarded as an earpiece integrating all parts of a hearing aid, a BTE hearing aid comprises a housing adapted for resting over the pinna of the user and an ear piece adapted for insertion into the ear canal of the user and serving to convey the desired acoustic output into the ear canal. The earpiece is connected to the BTE housing by a sound conduit or, in case it houses the receiver, by electric leads. In either case it has an output port for conveying the sound output.
2. Description of the Related Art
WO-A1-00/03561 provides an in-the-ear hearing aid wherein the acoustic outlet port is protected against contamination by earwax by means of an earwax guard, which is inserted in port. An elastic hose connects the port to a receiver. The earwax guard comprises an essentially tubular element with a through-going cavity and an abutment collar in one end for sealing abutment against an edge of the hearing aid housing adjacent the port.
EP-A2-1432285 shows a method for hydrophobic coating of components for a hearing aid, such as for the battery lid, the battery compartment, the housing or a switch.
DE-A1-102004062279 shows an earwax guard for a hearing aid, which has been provided with an oleophobic or biofilm-inhibiting coating.
EP-A2-1458217 shows an acoustic filter of a hearing instrument, detachably placed nearby or at the opening for the acoustic output of the instrument. The filtering element is made of a polymer material, a synthetic, metallic or ceramic material or a fabric-like material.
EP-A2-1432285 provides a method for hydrophobic coating of a hearing aid for the purpose of preventing entry of moisture into crevices and openings of the housing.
U.S. Pat. No. 3,354,022 provides a water-repellant surface having high and low portions with an average distance between high portions of not more than 1000 microns and an average height of high portions of at least 0.5 times the average distance between them; and having an air content of at least 60%. The air content of the surface is determined by taking an imaginary plane parallel to the surface passing through the tops of the high portions of the surface and measuring at this plane the percentage of the total surface area which is air. The surfaces may be coated with a solid having a water contact angle of greater than 90 degrees. These surfaces are highly water repellant.
WO-A1-0058415 provides a device for the loss-free transport or emptying of hydrophilic liquids, which device has raised areas and cavities on the side facing the liquid, the distance between the raised areas being between 0.1 and 200 microns and the height of said raised areas between 0.1 and 100 microns, and the raised areas being hydrophobic.
With a hearing aid or an ear piece having an output port inserted into the ear canal of a user there is a risk of earwax or moisture entering the port. The earwax may slowly accumulate or it may be driven into the port by the manipulation of inserting the hearing aid or the ear piece into the ear canal. The result is that the port clogs and baffles the acoustic output. For preventing this it is a standard practice to fit the output port with a replaceable earwax guard. The earwax guard incorporates baffles or a grid for establishing a barrier against the entry of earwax while permitting the passage of sound. The earwax guard may not be effective to entirely prevent the entry of moisture. The earwax may accumulate on the earwax guard. Once the earwax guard has been clogged, it is removed and replaced by a new one.
As far as pertains the microphone port, there may also be a risk of entry of moisture and earwax, although there may be less exposure to earwax as the microphone port faces the surroundings rather than the ear canal. A grid may be provided, although it may not be effective for protection against the entry of moisture.
With a hearing aid fitted with an earwax guard adapted for easy removal, there is the risk that the earwax guard accidentally is lost, or that the user removes it without inserting a new one, e.g. if he or she has no replacement available. When using the hearing aid without the earwax guard there is a risk of earwax entering deeper into the hose and ultimately into the receiver, where it may clog the receiver membrane or it may accumulate on the integral acoustic filter, if present. The same might happen if the earwax guard was not effective, i.e. if it was open for penetration of earwax. In either case, the outcome is a costly service operation, involving disassembly or replacement of the receiver. It is estimated that a major proportion of service issues with hearing aids is related to the entry of earwax or moisture into the output port.
Providing the receiver with an external acoustic filter complicates logistics. An acoustic filter normally serves to correct acoustic artifacts of the receiver. An acoustic filter works by absorbing acoustic energy, e.g. for dampening resonance peaks or otherwise shaping the frequency response. The acoustic filter must be tailored to the particular receiver in order to provide a satisfactory shaping with minimal loss of acoustic energy.
For logistic reasons it would be easier if a standard earwax guard could be used for all types of hearing aids. However, a standard earwax guard necessarily must be acoustically transparent in order not to absorb energy and possibly distort the desired acoustic output in a non-controlled way. The requirement for the filter being acoustically transparent runs against the consideration of the filter providing an effective barrier against earwax and moisture. Therefore general earwax guards may not be effective for preventing the entry of moisture.